1. Field of the Invention
The present invention is directed to an imaging sensor sensitive to thermal infrared radiation.
2. Description of the Prior Art
Current imaging systems need to obtain images from static, unchanging scenes. This is the case in most applications, not only in laser profiling. These types of systems require the modulation of the incoming radiation because the sensors, including pyroelectric arrays, only respond if the amount of radiation the sensors xe2x80x9cseexe2x80x9d in successive scans is different. If there is no change (or modulation of radiation) there is no response from the sensor.
This is generally accomplished by rotating a disc in front of the sensor. The disc provides an opening through which input scene radiation is allowed to pass into the sensor for the duration of a complete field of scanning. The remainder of the disc is solid and blocks all radiation for, again, the duration of a complete field of scanning. This provides the necessary difference between two successive scans in order to generate a useful image from the imaging sensor. A typical rotating disc 10 is shown in FIG. 1, provided with a darkened or closed portion 12, as well as an open portion 14. As shown in FIG. 1, the closed portion 12 generally constitutes half of the potential surface area of the rotating disc. However, the percentage of the darkened portion with respect to the opened portion can vary. The disc rotates in a synchronous manner such that, just after an open field of information is read from the imaging sensor, the solid or closed portion 12 of the disc covers that particular area of the sensor. This closing edge of the disc will continue to track the scanning of the imaging sensor for the duration of the entire field. Alternatively, the imaging sensor is re-scanned for each succeeding field of information that has been exposed only to the solid portion of the disc. When this occurs, the opening in the disc will soon arrive, restarting the two field cycle of one open-one closed field of information. This type of disc will provide the maximum sensitivity that the imaging sensor will have to offer.
Generally, any sensor is limited either by its damage threshold, or its useful dynamic range. Typical imaging systems involve the use of an objective lens. These lenses most often also have a variable aperture, namely, an iris. This iris will perform the function of increasing the dynamic range of an imager by decreasing the effective aperture of the lens in use, rather than the unwieldy changing to a lens of smaller aperture.
The laser industry calls for the use of an unobstructed path to an imaging sensor that will handle the laser""s full operating power. Laser information itself is collimated, thereby not requiring an objective lens. This means that an iris is not even an available option for this application. In addition, the imaging system desired cannot use neutral density filters (NDF""s) to reduce the radiation seen by the imaging sensor.
Prior art optical filters or choppers have been described in various U.S. patents, such as U.S. Pat. Nos. 4,165,919, issued to Little; 4,640,580, issued to Schlesinger; 5,255,117, issued to Cushman; and 4,581,515, issued to Egashira.
The patent to Little describes an adjustable optical filter provided with two filter wheel discs. The filter merely acts as an optical iris in that changing the relative position of one of two filter wheels with respect to the second wheel, changes the amount of sunlight that passes through a net opening. This net opening is related to the size of the open area remaining from the two wheels. These filter wheels do not need to spin, nor is there rotation in synchronism with any detector assembly noted. In addition, the patent describes use of a filter material, such as Mylar to help reduce maximum radiation.
The Schlesinger patent discloses the use of two rotating chopper wheels. These wheels are not located close to one another with each wheel required to be located on one side of a focal plane. Therefore, these wheels cannot be synchronized with a detector read-out or changed in phase or the amount of overlap. The purpose of this invention is to produce focus dither in an optical system.
The patent to Cushman discloses an advanced eye or sensor protection in a high-speed variable optical continuation system. This system includes protective shutters 14 and 16 shown in FIGS. 2 and 3. However, these shutters are not designed to be synchronized with one another to produce or to control the sensitivity of the system.
The patent to Egashira discloses a means for producing laser beam modulation by using rotating discs as choppers. The distribution of power density of the laser beam is altered by opening and closing a passage for the laser beam. Rotatable chopping disc 53, 55, 57 and 59 having circular holes are provided.
None of these patents are directed to a system for increasing the range of radiation illumination that a sensor can be subjected to, without the use of irises or NDF""s, or providing a maximum sensitivity.
The deficiencies of the prior art are overcome by the present invention which is directed to a system for modulating thermal infrared radiation for an imaging sensor. The system increases the range of radiation illumination that a sensor can be subjected to without using irises or NDF""s. Two parallel discs are provided and are disposed in between a radiation source and the sensor. Each disc has an opening which, in the illustrated embodiment, is approximately xc2xd the disc. Maximum sensitivity occurs when the openings in the two discs coincide, as the two discs rotate in synchronization with the scanning format of the imaging sensor. To reduce sensitivity, one disc is offset from the second disc so that its opening does not totally coincide with the opening of the second disc when both discs rotate at the same speed, thereby decreasing the total open area and reducing the amount of radiation received by the sensor. The amount of radiation can be varied from 50/50 to 1/99, or an infinitesimal fraction to provide a large dynamic range of input, thereby enhancing laser beam profiling. Systems that perform this function by adjusting the lens iris are more expensive and more dependent on the specific objective lens used. Systems that perform this function by adding neutral density filters are equally cumbersome to implement. These and other aspects of the present invention will become apparent to those skilled in the art after reading the following description of the preferred embodiments when considered with the drawings.